The present invention is directed to a brushless d.c. drive.
Brushless permanent-field d.c. drives may be used in motor vehicles for a variety of purposes, including electric power-assisted steering. These d.c. drives have a synchronous motor, which may have a star-connected stator winding or armature winding and a permanent-field rotor. The armature winding is connected to the direct voltage network by a converter in a bridge circuit having six semiconductor power breakers. The power inverter which causes commutation of the armature winding is controlled by an electronic controller. An example of a synchronous motor operated on a direct voltage network is discussed in German Published Patent Application No. 37 09 168.
If faults occur in the armature winding and/or in the power breakers, the d.c. drive may generate a permanent electromagnetic braking torque without a direct voltage being applied, if the synchronous motor operates as a generator against a low-resistance load impedance. In at least some applications, such a braking torque may have a negative effect on the functioning of the unit or system in which the d.c. drive is used. For example, in the case of electric power-assisted steering systems, the braking torque which occurs in the event of a fault may necessitate a considerable steering force being applied by the driver, which may be unacceptable. Accordingly, devices can be provided on such a d.c. drive to lead to a fail-silent response of the d.c. drive in the event of a fault, i.e., the d.c. drive does not have any interfering or negative effect on the unit or system, so the latter functions as if the drive were not present.
In an electric power-assisted steering system, a mechanical clutch, by way of which the output shaft of the synchronous motor acts on the steering gears, may be used to produce the desired fail-silent response. In the event of a fault, the clutch is opened to uncouple the motor from the steering system.
The exemplary brushless d.c. drive according to the present invention may have the advantage that the desired fail-silent response of the d.c. drive is achieved without any expensive external components, such as mechanical clutches, with simple circuitry measures in the drive itself. Thus, the d.c. drive becomes more compact and requires less space, so that it can be used in a more versatile manner. The additional cost incurred for the desired response of the d.c. drive in the event of a fault may be greatly reduced.
According to an exemplary embodiment of the present invention, the separating apparatus, arrangement or structure for separating the connections between the winding phases of the armature winding may be activated by a control unit which detects a fault case.
According to another exemplary embodiment of the present invention, the control unit has, for this purpose, measurement shunts in each connecting line between the armature winding and the switching device designed as a bridge circuit having semiconductor switches. In simultaneous blocking phases of all semiconductor switches, the electric currents flowing through the measurement shunts are measured, and in the event of a current value which differs significantly from zero in one of the measurement shunts, the control device delivers an activation signal to the separating apparatus, arrangement or structure. Such a design of the control unit with which faults occurring in the switching device are detected may have the advantage that the measurement shunts already present in the d.c. drive for measuring the current for other reasons can also be used to detect the fault case, thus further reducing the complexity of the circuitry. Faults in the armature winding itself can be detected, for example, by measuring the braking torque delivered to the output shaft of the synchronous motor. This may be an advantage in the case of electric power-assisted steering systems, since sensors for measuring torques on the input and output shafts are already provided in the final control elements of the electric steering devices.
According to yet another exemplary embodiment of the present invention, the control unit in a star connection of the armature winding includes measurement shunts, each connecting a winding phase of the armature winding to the neutral point. The control unit continuously measures the amount and phase of currents flowing through the measurement shunts and adds the shunt currents as vectors. In the event of a significant deviation in the result of this addition from zero, the control unit delivers an activation signal to the separating means. With such a control unit, faults in the semiconductor switching device as well as faults in the armature winding may be detected, and the separating apparatus, arrangement or structure is activated accordingly.
According to other exemplary embodiments of the present invention, the separating apparatus, arrangement or structure may cause a reversible or irreversible separation of the connections between the winding phases of the armature winding. An irreversible separation can be brought about by way of pyrotechnic blasting charges or by fusible cutouts. For reversible separation, electric contacts controllable by an electronic or mechanical apparatus, arrangement or structure are used. In the case of armature windings in a star connection, the neutral point is separated, but in the case of armature windings in a delta connection, each winding phase must be separated from the winding terminations.